Techniques for testing or analyzing optical components are known. A “device” under test (DUT), such as a length of fiber optic cable, may be carefully tested for faults or may be analyzed to determine whether the device is suitable for use in a particular application. System components such as multiplexers, demultiplexers, cross connectors, and devices having fiber Bragg gratings may be separately tested before a system is assembled.
Optical testing may be performed using a heterodyne optical network analyzer. Such analyzers may be employed for measuring properties of optical components, such as group delay. “Group delay” is sometimes referred to as envelope delay, since it refers to the frequency-dependent delay of an envelope of frequencies, with the group delay for a particular frequency being the negative of the slope of the phase curve at that frequency. Typically, a heterodyne optical network analyzer includes two interferometers. An example of a heterodyne optical network analyzer 10 having two interferometers 12 and 14 is shown in FIG. 1. A tunable laser source (TLS) 16 generates a laser light beam that is split by a coupler 18. The TLS is continuously tuned, or swept, between a start frequency and a stop frequency. By operation of the coupler 18, a first portion of the coherent light from the TLS is directed to the DUT interferometer 12, while a second portion is directed to the reference interferometer 14.
The DUT interferometer 12 has a second coupler 22 that allows beam splitting between a first arm 24 and a second arm 26. A mirror 28 is located at the end of the first arm and a DUT 20 is located near the reflective end of the second arm. The lengths of the two arms can differ, and the difference in the optical path length is represented in FIG. 1 by LDUT. Since the DUT can be dispersive, the actual optical path length is a function of frequency. A detector 30 is positioned to measure the combination of the light reflected by the mirror 28 and the light reflected at the DUT 20. Processing capability (not shown) is connected to the detector 30 to measure group delay of the DUT as a function of frequency. However, in order to very precisely measure the group delay, it is necessary to obtain knowledge of the frequency tuning of the TLS 16 as a function of time. The reference interferometer 14 is used for this purpose.
The structure of the reference interferometer 14 is similar to that of the DUT interferometer 12, but a mirror 32 takes the place of the DUT 20. A second detector 34 receives light energy that is reflected by the combination of the mirror 32 at the end of a third arm 36 and a mirror 38 at the end of a fourth arm 40. As in the DUT interferometer, the lengths of these two arms can be different, and this difference in lengths is represented by LREF. The optical characteristics of the reference interferometer are fixed and known.
A potential problem occurs in the heterodyne optical network analyzer 10 when the path length difference (LDUT) is sufficiently large that coherence effects become an issue. The frequency generated by the TLS 16 undesirably fluctuates in a random manner around its target frequency as it is tuned. The random fluctuations occur as a result of various quantum or stochastic effects. The random fluctuations of the frequency affect the frequency of the heterodyne interference signal measured by each detector 30 and 34. When the group delay of the DUT 20 is calculated, the frequency fluctuations of the TLS 16 manifest themselves as noise in the group delay measurement. This ultimately limits the precision of the measurement process. This effect is referred to as “phase noise.” The phase noise on the measurement process increases as the path length mismatch for the two arms 24 and 26 of the DUT interferometer 12 increases, until the path length mismatch equals or exceeds the coherence length of the laser beam.
What is needed is a method and system for at least reducing the deleterious effects of phase noise in an interferometric system.